Use of both Invisible and Emergable Watermarks to Deter Illegal Copying of Images

Emergable watermarks become visible in content that has been copied under designated conditions so that their use should deter illegal copying. However, the message presented by an emerged watermark is easily removed from the copied content because it is visible to the copier. One way to overcome this problem is to also use invisible watermarks which are robust against removal. However, mutual interference caused by embedding two different types of watermarks would degrade the performance of both types. A method has thus been developed in which frequency filtering is used to divide the content into two types of region, one for embedding an emergable watermark and one for embedding invisible one. The emergable one is embedded in a plain large region with low spatial frequency while the invisible one is not. Experimental evaluation demonstrated that the image quality of an image with watermarks embedded by using the proposed method was improved better than that of an image watermarked using the previous method for embedding only emergable watermarks.